During operation of a gas-discharge lamp such as a high-intensity discharge lamp, the high temperatures of the electrodes embedded in the pinch regions lead to correspondingly high levels of stress owing to the different rates of expansion and contraction of the quartz glass and the electrodes, which are usually made of tungsten. These stresses lead to the development of cracks, of which there are several different kinds For example, cracks can develop in a region close to the discharge chamber, or in a region between the electrode and a molybdenum foil embedded in the pinch, or close to the cut end of the pinch at the point where the body of the lamp is detached from a carrier quartz body in a final manufacturing step, referred to as “cutting-edge cracks”, etc. Much effort has been invested in lamp designs that strive to minimize the development of cracks, since these can lead to lamp failure. For example, some designs include longitudinal grooves along the outer surface of the electrode in the region enclosed within the pinch. These grooves should prevent the development of cracks close to the discharge chamber, where the temperature of the electrode is highest. Other designs deliberately leave a gap between the quartz and the electrode body so that the electrode can freely expand and contract. Alternatively, some designs make use of a “hairbrush” structuring in a region of the electrode close to the discharge chamber, to counteract the high temperatures nearer the discharge end of the electrode and to discourage the development of a radially extending crack (REC) that would lead to the failure of the lamp. However, any structuring carried out on the surface of the electrode may compromise its mechanical flexibility and capacity to deform, and excessive structuring may in turn lead to electrode failure.
Another type of crack may develop close to the end of the electrode and is referred to as an “end-of-electrode crack” (EEC). An EEC typically travels axially from a point near the base or outer end face of the electrode, in a region where the electrode is bonded to a conductive foil, towards the outer end of the pinch. An EEC typically has a curved shaped in the manner of a flat parabola. The development of such EECs is not yet fully understood. There may be different causes that trigger the development of such a crack. Stress in the pinch during operation of the lamp, particularly an inhomogeneous stress distribution is believed to be a major contributing factor and can promote or accelerate EEC growth. The inhomogeneous stress distribution could be a result of variations in material interactions. In any case, it may be assumed that some kind of nucleus encourages an EEC to develop. Such a “nucleus” could be generated by different mechanisms, it could be induced chemically or mechanically, e.g. by an impurity in the quartz glass in the region around the conductive foil, or by a “micro-crack” that has developed in that region as a result of an unfavourable distribution of thermally-induced tensile and compression stresses. Alternatively, a cutting-edge crack may act as a nucleus to trigger an end-of-electrode crack. Regardless of the mechanics of its development, once an EEC appears, lamp failure is imminent, so that it is of great importance to inhibit its development in the first place.
Therefore, it is an object of the invention to provide an improved electrode design that avoids the development of an end-of-electrode crack.